1. Field of the Invention
The present invention is directed to an electronic multilevel package which includes a poly(aryl ether benzimidazole), a polyimide and copper and a process for making same. In addition, the present invention is directed to new species of poly(aryl ether benzimidazoles) which are employed in the formation of multilevel electronic packages.
2. Background of the Prior Art
Electronic packaging is commonly employed in electrical and electronic devices, assemblies, subassemblies, systems and the like. Usually, electronic packaging involves a multiplicity of layers, each layer of which includes one or more electrical circuits. Most commonly, a circuit or circuits is formed by copper wiring or metal deposition disposed on a layer of a polyimide which, in turn, is disposed upon a substrate.
In forming multilayer electronic packages a first polyimide precursor is disposed upon a substrate. That layer is thereupon cured to a polyimide. A patterned copper layer, defining an electrical circuit is embedded into the polyimide cured layer. The copper layer may be raised above or at the height of the polyimide where it is referred to as raised or planar, respectively. This procedure is repeated to form the multilayered package.
Although the use of these materials are most appropriate to this application, there are problems associated with their use. For one thing adhesion between copper and polyimides is poor. For another, copper reacts with polyimide precursors to form a fine precipitate of copper oxide particles within the polyimide layer. These two problems can be disastrous in electronic application insofar as both of these problems can lead to failure of the circuit. That is, poor adhesion can easily lead to discontinuity in the copper circuit based on the dislodging of all or part of the copper conduit paths. Similarly, the inclusion of free copper oxide particles increases the electrical conductivity of the polyimide which also leads to malfunction.
Many proposed solutions have been advanced to overcome these well understood problems. One such solution, advanced to overcome the reactivity and poor adhesion between copper and polyimides problems, is to contact these materials with chromium.
When chromium is employed in a copper-polyimide planar layer, a chromium layer is disposed thereover which prevents contact between the copper on the first layer with the polymide in the second layer. This results in a chromium-polyimide interface and because, as those skilled in the art are aware, there is good adhesion between chromium and a polyimide, the adhesion problem is alleviated. Moreover, chromium does not react with polyimides. As such, no reaction product of a chromium-containing compound is formed to disturb the electrical insulation between the layers.
Although this expedient is effective it is time consuming and expensive. It is even more time consuming and expensive when raised copper layers are involved. To cover the copper-polyimide layers, chromium must be sputter coated to provide a thickness of 200.ANG. to 500.ANG. followed by coating thereupon of the next polyimide layer. As those skilled in the art are aware, this involves the laying down of a film of a polyamic acid which thereupon is cured to form the polyimide.
An alternative to coating with chromium is to coat with tantalum. However, this change merely represents the substitution of one metal for another. The time duration, the complexity and the expense of this operation is unchanged.
Yet another solution is provided by the introduction of an electroless metal, usually nickel, to coat the copper and provide a barrier to its contact with the polyimide layer. This coating operation is technically difficult and is therefore costly. Moreover, the use of an electroless metal coating is oftentimes associated with an additional coating step, the coating of a silane coupling agent, which increases the complexity and expense of this alternative.
Other methods of overcoming the difficulties of copper-polyimide contact in multilevel electronic packages are provided by U.S. Pat. No. 3,770,573 to Lindsey and U.S. Pat. No. 3,361,589 to Dumphy et al. These patents, which are incorporated by reference, describe methods of chemically treating the surface of polyimide film layers which improve their adhesivity.
Other proposed solutions have been advanced. Suffice it to say, these solutions involve the use of adhesion promoting agents which suffer from incompatibility between their thermal, mechanical and/or electrical properties and those of the polyimide layers with which they are in contact.
For example, U.S. Pat. No. 4,048,005 to Nakagome et al. describes a process for producing a heat resistant laminated metallic sheet composed of mutually insulated outer metallic sheets or foils disposed between an inner binder layer of a film of a heat resistant heterocyclic polymer from which the volatile matter is substantially removed. The heterocyclic polymer may be a polyimide, a polyamide-imide, a polybenzimidazole, a polyhydantoin, a polyparabanic acid, a polythiazole or a polyimidozopyrrolidone.
Not only does the generalized nature of the polyimides within the contemplation of Nakagome et al. not specifically describe the particular polyarylene ether benzimidazoles of the present invention but, moreover, there is no disclosure of utilizing the polymers of this patent as an adhesive layer disposed between copper circuitry and an adjacent polyimide layer.
U.S. Pat. 5,180,639 to Zarnoch describes a method of modifying an aromatic polymer surface to improve the adhesion of that surface to a metal layer thereon. As such, this method is similar to the concept that has been suggested for overcoming the problems associated with poor adhesion between copper and polyimide layers.
Although the Zarnoch patent involves a method of modifying an aromatic polymer surface to improve the adhesion to a metal layer, the aromatic polymers described in Zarnoch are primarily polycarbonates which are subjected to nitration. The complexity of including chemical reaction, the nitration step, in the formation of the modified surface makes this process of this patent unfavorable.
U.S. Pat. No. 4,022,649 to Nakagome et al. discloses a method of producing a metal laminate which includes a polybenzimidazole layer. Although this patent deals with heterocyclic polymers of the type utilized in the present invention, the polymers utilized in the '649 patent are not used to cap the copper layer to prevent copper from reacting with polyamic acid. Thus, the '649 patent does not provide a teaching which results in enhanced adhesion of copper to polyimide.
U.S. Pat. No. 5,120,819 to Lubowitz et al. sets forth a class of crosslinkable oligomers that include oxazole, thiazole or imidazole linkages. It is emphasized, however, that the '819 patent does not disclose or suggest the use of the oligomers taught therein in the capping of copper. Neither does it disclose or suggest a process of manufacturing a multilayered electronic package.
U.S. Pat. No. 3,533,879 to Levine describes a process wherein fusible polybenzimidazoles impregnate fabrics permitting the formation of fabric laminates and the like. This patent provides no disclosure relating to an electrical multilayered article.
U.S. Pat. No. 3,957,726 to Gordon et al. discloses novel polyimides which polymers are described as useful in the formation of prepregs and laminates. None of the polymers, prepregs, films and laminates, disclosed as useful in the formation of prepregs and laminates, are described in Gordon et al. for use in the capping of copper, essential in the formation of multilayered electrical packages.
The above remarks establish the need in the art for a new multileveled electronic package and a process for forming the same, which insures adhesion of copper and which also is free of the problem associated with the formation of copper-containing precipitates, to thus minimize degradation of the dielectric in multilayered electronic packages.